1. Field of the Invention
The present invention relates to a method for transmitting a synchronization channel using multi-antenna and, more particularly, to a method for transmitting a synchronization channel in order to enhance a diversity performance (or capability) of a Broadcast Channel (BCH) or control channel.
2. Discussion of the Related Art
A synchronization channel is referred to differently depending upon the respective system. For example, in the 3GPP LTE, the synchronization channel is referred to as a Synchronization Signal (SS), and, in the IEEE802.16e, the synchronization channel is referred to as a preamble. Therefore, the synchronization channel collectively refers to a channel and/or signal used by a user equipment (or terminal) for performing time and/or frequency synchronization with a base station.
Generally, a base station transmits a signal through a plurality of antennae (i.e., more than one antenna). Herein, the number of antennae used may be indicated to the user equipment through a synchronization channel, or a broadcast channel (hereinafter referred to as “BCH”), a control channel, a data channel, and so on. However, at a time point when the user equipment initially performs synchronization with the base station, the base station is unable to determine the number of channels that have been used for the transmission. Accordingly, in this case, the base station may transmit a synchronization channel to the user equipment by adopting a transparent transmission method. Examples of the diversity method, which may be performed without knowing the number of antennae transmitted by the user equipment, may include Time Switched Transmit Diversity (hereinafter referred to as “TSTD”), Precoding vector Switching (hereinafter referred to as “PVS”), Cyclic Delay Diversity (hereinafter referred to as “CDD”), and so on.
Hereinafter, an example of transmitting a synchronization channel through 2 antennae when performing TSTD will be given in the following description for simplicity. In this example, it is assumed that the synchronization channel is transmitted at a cycle period of 10 ms. Accordingly, at the time point t=T0, the synchronization channel is transmitted only through Antenna 0 (herein, Antenna 1 is turned off). At the time point t=T0+10 ms, the synchronization channel is transmitted only through Antenna 1. And, at the time point t=T0+20 ms, the synchronization channel is transmitted only through Antenna 0. In other words, TSTD refers to a method of transmitting a synchronization channel by alternating antennae at different time points. By using this method, Time Diversity Gain may be obtained by using a time variant channel characteristic of TSTD. And, since it may appear to the user equipment as though the base station performs transmission by using only one antenna, the user equipment may be capable of detecting the transmitted signal without difficulty, even without knowing the number of antennae used by the base station to perform transmission.
Hereinafter, an example of transmitting a synchronization channel through 2 antennae when performing PVS will be given in the following description for simplicity. In this example, it is assumed that the synchronization channel is transmitted at a cycle period of 10 ms. Accordingly, at the time point t=T0, a transmission signal is multiplied by a precoding vector of [+1 +1]T and then transmitted. More specifically, in Antenna 0, the transmission signal is multiplied by 1 and then transmitted, and, in Antenna 1, the transmission signal is multiplied by 1 and then transmitted. Also, at the time point t=T0+10 ms, a transmission signal is multiplied by a precoding vector of [+1 −1]T and then transmitted. More specifically, in Antenna 0, the transmission signal is multiplied by 1 and then transmitted, and, in Antenna 1, the transmission signal is multiplied by −1 and then transmitted. Therefore, by using this method, PVS may vary the channels through time-based precoding, thereby acquiring Time Diversity Gain even in a low mobility environment. And, since it may appear as though the base station performs transmission to the user equipment by using only one antenna (i.e., MS transparent), the user equipment may be capable of detecting the transmitted signal without difficulty, even without knowing the number of antennae used by the base station to perform transmission.
Hereinafter, an example of transmitting a synchronization channel through 2 antennae when performing CDD will be given in the following description for simplicity. In this example, it is assumed that the synchronization channel is transmitted at a cycle period of 10 ms. Herein, different delays respective to each antenna are applied to the time domain signal, which is received through each antenna. Then, the processed signal is transmitted. At this point, the signal prior to being applied with the delay transmits identical signals between each antenna. This increases the size (or amount) of a multi-path, thereby increasing a frequency selectively of the corresponding channel. Accordingly, the receiving end may detect a signal by using the same method as when using only one antenna regardless of the number of antennae used by the transmitting end. More specifically, the user equipment is not required to know the delay value applied by the transmitting end. However, this corresponds to when a small delay within a number sample is applied. In other words, when a large delay corresponding to one-half (½) or one-quarter (¼) of an Orthogonal Frequency Division Multiplexing (OFDM) symbol length is applied, it is preferable that the user equipment is informed in advance of the applied delay value.
Generally, cell identification (ID) information may be carried in the synchronization channel so as to be transmitted. In other words, the synchronization channel is carried in a time/frequency/code unique to each cell, thereby being transmitted. Therefore, after performing the initial synchronization, when demodulating a signal such as a broadcast channel (BCH) or data, a channel may be estimated by using a synchronization channel instead of a pilot, or by combining a pilot and a synchronization channel.
Since the synchronization channel has a greater density that a general pilot channel in the frequency domain, the channel estimation performance may be largely enhanced. Hereinafter, in order to simplify the description of the present invention, the BCH will be described as an example of a channel that can demodulate data through the synchronization channel.
A Space Frequency Block Code (SFBC) corresponds to a branch of rank-1 transmission among many multi-antenna transmission methods. Herein, the Space Frequency Block Code (SFBC) refers to a method of obtaining diversity gain by multiplying the transmission signal by the SFBC. This method corresponds to a method applying a Space Time Block Code (STBC) proposed by Alamouti to the frequency domain.
FIG. 1 illustrates a block view of an STBC transmission.
When M_rx corresponds to a number of receiving antennae, SFBC applies the row of a 2×M_rx matrix of an STBC encoder as the frequency domain.
Generally, in light of spatial diversity in a rank-1 transmission, it is known that SFBC (or STBC) is yields its best performance within the same environment. In other words, SFBC ensures a more outstanding performing as compared to other rank-1 transmissions, such as TSTD, PVS, CDD, and so on. Nevertheless, in order to perform decoding, channel estimation for each transmitting antenna is required to be performed.
In the related art method, the TSTD/PVS/CDD is used in the synchronization channel, and TSTD/PVS/CDD completely identical to that applied to the synchronization channel is used in the BCH. Herein, the synchronization channel should be used as a phase reference or an amplitude reference for BCH decoding. However, if an antenna transmission method such as TSTD/PVS/CDD is used in the synchronization channel, the same TSTD/PVS/CDD should be used in the BCH. And, accordingly, since channel estimation cannot be performed for each antenna, SFBC yielding the most optimal performance cannot be used.